What is it about muting per the manual you consider too complex? Are you wanting to mute just the audio, or mute the entire receiver during transmit? What control voltages and polarity do you want, or have now?
It's really pretty easy to do, either way. If you're wanting to both disable the front end and mute the audio its a little more involved but doable without too much effort.
N2ROC TOLD ME THE EASYEST WAY TO DO IT. I PUT THE STANDBY SWITCH IN STBY AND USE THE TERMINALS ON THE BACK TO MUTE BY USING THE STBY AND GND. IT WORKS. WHEN I TRANSMITT I WANT THE RECEIVER TO GO INTO MUTE, BUT THE MANUAL TALKS ABOUT A -22VOLTS THAT I HAVE TO GET FROM THE TRANSMITTER.
On page 4 of the NC-300 manual, Section 2-4, and Figure 2 on page 5, explain the same thing you did. If you wanted to use the grid-block method, you use the -22 volts. It's all there in the manual. No mystery.